It is known that a variety of chemical substances absorb light in proportion to the concentration of the substance present in the sample. Furthermore, the light transmitted through such a substance has an absorption spectrum characterized by the light absorbing properties of the substance and the properties of any other medium through which the light travels. Such absorption spectrum can be prismatically revealed for analysis. By discounting the portion of the absorption spectrum attributable to intensity losses and other absorbers, the spectrum of the chemical substance can be isolated and its identity and concentration determined. The discounting, or “referencing,” is done by determining the absorption spectrum of the light source and any spectrophotometric components in the absence of the chemical substance. Referencing is usually done close in time and space to the measurement of the absorbance of the chemical substance to minimize error.
It is well known that portable, battery-powered devices for determining the concentrations of chemical substances are commercially available. Examples include portable photometers provided by Hach Company and portable reflectometers by Merck. A detailed review of photometric and reflectometric systems is given in Comprehensive Analytical Chemistry, Chemical Test Methods of Analysis, (Y. A. Zolotov et al., Elsevier, New York (2002)), and in a review paper given in Review of Scientific Instruments, (Kostov, Y. and Rao, G., Vol. 71, 4361, (2000)). The adoption of these systems makes chemical analysis outside of a laboratory possible. However, improvements in the following areas are still needed:                1. Some tests with portable instruments use toxic or corrosive reagents.        
Some use a large quantity of solid reagents for a single test. For example, many Hach test methods use 200 mg or more solid reagent for a single analyte.                2. An operator has to transfer reagents and sample into a measuring unit.        
Sample manipulation and reagent handling are inconvenient parts of chemical analysis and multiply operator-to-operator errors.                3. Liquid waste product resulting from the wet chemistry analysis has to be safely disposed according to applicable laws.        4. Currently available test methods cannot easily determine more than one unrelated analyte in a single test.        5. Although most portable devices have data interpretation and storage capabilities, most test results still need to be transferred manually into a database.        
Other methods utilizing test strips have been widely attempted for semi-quantitative analysis for a large number of analytes. Here, quantitative results can be obtained with disposable optical sensor elements, read by a photometer. In most instances, only a single analyte is determined by an optical sensor element. Since transmission absorbance is measured, it is difficult to produce disposable optical sensor elements for calibration free tests.
Disposable chemical sensors are well known in the art. For example, U.S. Pat. No. 5,830,134 describes a sensor system for detecting physico-chemical parameters designed to compensate for numerous perturbing factors, such as those resulting from the use of partially disposable monitoring units, thus eliminating the need for calibration steps.
Another U.S. Pat. No. 5,156,972 discloses a chemical sensor based on light absorption, light emission, light scattering, light polarization, and electrochemically and piezoelectrically measured parameters.
Scatter controlled emission for optical taggants and chemical sensors have been disclosed in U.S. Pat. No. 6,528,318.
Sensor arrays that use reference and indicator sensors are known and described in U.S. Pat. No. 4,225,410. Here, a sensor can be individually calibrated, such that each analysis can be read directly.
U.S. Pat. No. 5,738,992 discloses a method that utilizes a reference material to correct fluorescence waveguide sensor measurements. U.S. Pat. No. 5,631,170 teaches a referencing method for fluorescence waveguide sensors by labeling the waveguide with a reference reagent. It should be pointed out that the internal absorbance standard method used in this invention is fundamentally different from the prior arts in several aspects.
First, the multiangle scatter-induced absorbance detection scheme used in the present invention is different from traditional Attenuated Total Reflection (ATR) sensors that use a thin element with the film thickness approximately the same size as the incident beam wavelength. These thin elements can also include a fluorophore that acts as internal references. In contrast, the present system pertains to thicker film elements that do not require thickness near the incident beam wavelength, and that use alternate internal references based on absorbance.
Two-wavelength, or dual-beam, methods are known in spectrophotmetric analysis. In “Referencing Systems for Evanescent Wave Sensors,” (Stewart, G. et al., Proc. Of SPIE, 1314, 262 (1990)), a two-wavelength method is proposed to compensate for the effect of contamination on the sensor surface. U.S. Pat. No. 4,760,250 to Loeppert describes an optoelectronics system for measuring environmental properties in which feedback-controlled light sources are used to minimize problems associated with the light source stability and component aging. A similar feedback-controlled two-wavelength method is described in U.S. Pat. No. 3,799,672 to Vurek. A dual-beam reflectance spectrophotometer is described in “Optical Fiber Sensor for Detection of Hydrogen Cyanide in Air,” (Jawad, S. M. and Alder, J. F., Anal. Chim. Acta 259, 246 (1991)). In Jawad and Alder's method, two LED's are alternately energized. The ratio of outputs at the two wavelengths is used to reduce errors caused by the background absorption of the sensor element for hydrogen cyanide detection. These two-wavelength methods are effective to minimize errors caused by optical and mechanical component aging and long-term stability problems of light sources. However, errors associated with variations in the effective optical pass length of disposable test elements have not been solved.
A disposable sensor system comprising a discardable or disposable measuring device and further comprising one or more sensors is disclosed in U.S. Pat. No. 5,114,859.
Furthermore, analysis of multiple analytes is done with microfabricated sensors as described in U.S. Pat. No. 6,007,775.
In “Application of a Plastic Evanescent-Wave Sensor to Immunological Measurements of CKMB,” (Slovacek, R. E.; Love, W. F.; Furlong, S. C., Sensors and Actuators B, 29, pp. 67-71, (1995)), it was demonstrated that a sensor handled by non-critical surfaces could be made with improved robustness. These sensing elements were fabricated as blunt-ended plastic cones onto which the sensing chemistries were deposited. The sensing elements were injection-molded from the plastic, making them commercially attractive.
Overall, the known existing sensors have several prominent shortcomings that limit their applicability for field analysis applications. These shortcomings include:                1. Need for critical alignment of testing strip in the sensor to perform accurate reading.        2. Need to reduce errors caused by variations in testing strip quality (imbedded reagent concentrations, effective optical path length, and component aging).        3. Need to reduce errors caused by physical changes in testing elements when they are exposed to a sample, such as swelling, shrinking, or/and crazing.        4. Need for determination of steady-state response in chemical sensor response for accurate analysis.        5. Inability to collect dynamic sensor information from nonreversible chemistries.        6. Inability to collect real-time information from nonreversible chemistries upon exposure to a sample.        7. Inability to analyze the dynamic sensor information from multiple nonreversible chemistries to provide an improved quantification ability of the sensor system.        
Because of the above shortcomings in the prior art, a low cost, handheld, and calibration-free sensor system has not been demonstrated. The sensor system disclosed in the present invention is directed toward solution of the above outlined shortcomings. In particular, the sensor in the present invention can collect dynamic information by tracking the rate of change of the kinetic or dynamic response of the non-reversible sensor chemistries as the sample reacts with the sensor in order to quantify the concentration level.
In view of the foregoing, it is an object of the present invention to provide a portable, disposable handheld sensor system for the quantitative determination of analyte concentrations. It is also desirable to provide a system that does not require calibration before each new set of analysis. In this regard, the present system employs dual light analysis on the same sensor element, where sample response is compared with an internal reference, eliminating the need for calibration before each new set of analysis. Moreover, the use of an internal reference significantly reduces the optical and mechanical coupling requirements for the device, thereby providing cost advantage in the manufacturing and assembly process with minimal impact on the accuracy of testing results
It is a further object of this invention to provide a sensor that is capable of communicating with an information processing unit, for example a pocket personal computer or wireless mobile phone or a satellite, so that analytical data can be manipulated, transmitted, or stored electronically.
It is important to note that the present invention provides a general photometric and/or spectroscopic test method where no liquid reagent is needed. This not only simplifies the test, but also reduces costly and labor-intensive requirements related to the handling and disposal of toxic reagent material.